The production of lithographic plates is well-known in the graphic arts industry. Lithographic processes generally employ at least two steps to prepare a plate for use in a printing press, namely exposure of a plate coated with a photosensitive composition to a light source through a mask (e.g., a positive or a negative mask), thus creating a photoimage, and a subsequent development step. Presensitized plates which are based on aluminum or polyester sheet-like material and are considered planographic in nature are commonly used in the industry. Typically, in a negative-acting system, the exposed areas become insolubilized (e.g., polymerized or crosslinked), and the development step involves washing material away from the unexposed portions of the plate. Conversely, in a positive-acting system, the development step involves washing material away from the exposed portions of the plate. The development step usually involves rinsing and washing with a developer solution and often is performed in a processing unit containing the developer solution. The developer solution generally is highly alkaline and often contains an organic solvent, such as benzyl alcohol. Alternatively, development of the photoimage has sometimes been accomplished by heating.
Both of the above-described (i.e., wet and thermal) development processes suffer from the drawback of being relatively time-consuming and expensive in some instances. Furthermore, when volatile organic or strongly alkaline solutions are employed as developer solutions, disposal of these solutions presents an environmental problem.
A printing plate which does not require processing prior to press would be desirable to the printing industry not only for environmental reasons, but also because it would reduce cost, reduce pressroom space, and decrease make-ready time. Because of these issues, the industry has long sought photosensitive compositions which do not have the above-described drawbacks. One method of overcoming these drawbacks is to provide a lithographic printing plate which does not require development or processing between imaging and being run on press. The term "no-process" as used herein refers to a printing plate which does not require any treatment, mechanical or chemical, between exposure and being run on press.
Constructions of positive-acting plates which do not require processing after imaging and before being run on press have been disclosed (e.g., see U.S. Pat. Nos. 5,102,771; 5,225,316 and 5,314,785). These patents disclose printing plates which are based on a photosensitive coating containing an ester of acrylic or methacrylic acid polymer and a photoacid-generating system. Upon imagewise exposure to actinic radiation, the polymer decomposes in the light struck regions, becoming fountain receptive, while areas in the non-light struck areas remain ink receptive.
Presensitized negative-acting lithographic plates which do not require processing after imaging and before being run on press are also known in the art. A category of plates which do not require processing between imaging and being run on press are described in U.S. Pat. Nos. 3,231,378; 3,285,745; and 3,409,487. These plates contain a photosensitive composition comprising a phenolic resin, an ethylene oxide polymer, and a photosensitizer. Upon exposure to actinic radiation, radicals are generated which cause the oxidation of the phenolic resin, thereby increasing the ink receptivity. A drawback of these plates is their tendency to become less ink receptive during the printing process. The loss of ink receptivity requires tight control of ink/water balance on press and, additionally, can cause the plates to become inoperable. U.S. Pat. No 3,793,033 discloses the incorporation of hydroxyethylcellulose ether in the construction. The resultant plates did not suffer from blinding (i.e. loss of image on printing) and required less control of ink/water balance on press. A significant disadvantage of this construction is the use of phenolic resins which require curing to achieve suitable run lengths on press. The curing requirements necessitate a two layer system with the sensitizer applied on top of the phenolic-containing layer after the phenolic layer has been cured. If the sensitizer is not coated after curing, it can decompose during the cure step.
Another category of negative-acting plates which do not require processing between imaging and being placed on press are those based on multiple photosensitive layers. Typically, these constructions have a hydrophilic photosensitive layer nearest the substrate and an additional layer which is capable of photohardening in conjunction with the first layer, as described, for example in U.S. Pat. Nos. 4,600,679 and 4,104,072 and European Pat. Publ. No. 450,199. Constructions such as these generally employ a negative-acting photosensitive composition based on aromatic diazo compounds as the hydrophilic layer closest to the substrate. A disadvantage is that these plates have a tendency to scum (i.e. take ink in the background areas) and also to contaminate the printing press. This drawback requires that plates first be washed with a suitable washing agent (i.e. fountain) prior to being run on press. Therefore, the incorporation of diazo compounds and resins is deleterious to the performance of plates intended for use as "no-process" plates. Printing plates are disclosed in PCT Application No. 93-05446 in which the diazo components have been eliminated from the hydrophilic layer, thus eliminating the problem of press contamination associated with residual unreacted diazo materials. The described plate is comprised of a plate/base substrate, a photosensitive polymeric hydrophilic layer nearest the base, and a separate photosensitive polymeric hydrophobic layer being next closest to the substrate. The photosensitive hydrophilic layer is based on polymerizable materials which do not contain diazo compounds. The functionality of the plate is derived from the insolubilization and hardening of both layers upon exposure to actinic radiation. This plate only requires a press equipped with ink and fountain to achieve good prints after exposure without the need to pre-wash the plate prior to being run on press. Because this construction has an external hydrophobic layer, it is impractical to achieve roll-ups equal to conventional plates without some treatment to remove the hydrophobic material from the background areas prior to being run on press.
It was against this background that the present invention dealing with negative-acting lithographic printing plates which require no further processing steps after exposure and prior to being placed on press was developed. These plates are prepared from a photosensitive composition comprising a reactive, acid functional polymer.
Reactive, acid functional polymers for a processed flexographic application have been disclosed in U.S. Pat. No. 5,268,257. In the preparation of flexographic printing plates, a relatively thick coating (typically 1-2 mm) of a flexible, photosensitive polymer is applied to a flexible substrate. During processing, a relief image is formed on the plate through exposure and wet development of the plate. The flexible, reactive, acid functional polymers disclosed in U.S. Pat. No. 5,268,257 are prepared by reacting a flexible carboxylated copolymer with a photosensitive vinyl containing compound, followed by an optional maleation reaction to increase the level of acid group functionality. Polymers prepared by this method result in a maximum acid group functionality of less than 0.37 ephr (equivalents per hundred gram of resin) and a maximum vinyl group functionality of less than 0.13 ephr.